The processing and handling of mailpieces consumes an enormous amount of human and financial resources, particularly if the processing of the mailpieces is done manually. The processing and handling of mailpieces not only takes place at the Postal Service, but also occurs at each and every business or other site where communication via the mail delivery system is utilized. That is, various pieces of mail generated by a plurality of departments and individuals within a company need to be collected, sorted, addressed, and franked as part of the outgoing mail process. Additionally, incoming mail needs to be collected and sorted efficiently to ensure that it gets to the addressee in a minimal amount of time. Since much of the documentation and information being conveyed through the mail system is critical in nature relative to the success of a business, it is imperative that the processing and handling of both the incoming and outgoing mailpieces be done efficiently and reliably so as not to negatively impact the functioning of the business.
In view of the above, various automated mail handling machines have been developed for processing mail (removing individual pieces of mail from a stack and performing subsequent actions on each individual piece of mail). However, in order for these automatic mail handling machines to be effective, they must process and handle "mixed mail." The term "mixed mail" is used herein to mean sets of intermixed mailpieces of varying size, thickness, and weight. In addition, the term "mixed mail" also includes stepped mail (i.e. an envelope containing therein an insert which is smaller than the envelope to create a step in the envelope), tabbed and untabbed mail products, and mailpieces made from different substrates. Thus, the range of types and sizes of mailpieces which must be processed is extremely broad and often requires trade-offs to be made in the design of mixed mail feeding devices in order to permit effective and reliable processing of a wide variety of mixed mailpieces.
In known mixed mail handling machines which separate and transport individual pieces of mail away from a stack of mixed mail, the stack of "mixed mail" is first loaded onto some type of conveying system for subsequent sorting into individual pieces. The stack of mixed mail is moved as a stack by an external force to, for example, a shearing device or nudger. The shearing device applies a force to the lead mailpiece in the stack to initiate the separation of the lead mailpiece from the rest of the stack by shingling it slightly relative to the stack. The shingled mailpieces are then transported downstream to, for example, a separating device which completes the separation of the lead mailpiece from the stack so that individual pieces of mail are transported further downstream for subsequent processing. In the mailing machine described immediately above, the various forces acting on the mailpieces in moving the stack, shingling the mailpieces, separating the mailpieces and moving the individual mailpieces downstream often act in a counterproductive manner relative to each other. For example, inter-document stack forces and normal forces exist between each of the mailpieces that are in contact with each other in the stack. The normal forces are created by the stack advance mechanism. The inter-document forces are created by the frictional forces between the documents, and potentially electrostatic forces that may exist between the documents. The inter-document forces tend to oppose the force required to shear the lead mailpiece from the stack. Additionally, the interaction of the force used to drive the shingled stack toward the separator and the forces at the separator can potentially cause a thin mailpiece to be damaged by being buckled as it enters the separator. Furthermore, in a conventional separator, there are retard belts and feed belts that are used to separate the mailpiece from the shingled stack. Both the forces applied by the retard belts and the feed belts must be sufficient to overcome the inter-document forces previously discussed. However, the force of the retard belts cannot be greater than the force of the feed belts or the mailpieces will not be effectively separated and fed downstream to another mail processing device. Moreover, if the feeding force being applied to the mailpieces for presenting them to the separator is too great, another potential problem which may occur is that a plurality of mailpieces will be forced through the separator without the successful separation of the mailpieces.
Another condition that affects the feeding of mailpieces is vertical orientation of the stack of mixed mail. The preferred orientation in which the most successful feeding occurs, is when mail is leaning slightly against the a paddle of the stack feeding device away from the nudger wall. When the mail is in this orientation, stack forces created by the weight of the mail in the stack are very low and the mail is more easily separated and aligned for feeding downstream into, for example, a separating device. The high stack forces are created by improperly loaded mail stacks or mail stacks that have shifted creating improper lean. The shifting can be caused by the inertia of the stack as it advanced, incrementally, to the shingling device. The high forces of the stack may also cause damage to mailpieces as they are fed out of the stack and can cause greater wear on the nudger or feed rollers. The high stack forces can also cause multi-feeds. Additionally, improper alignment of leaning mail along the feedpath to the separating device can cause the mail to skew as it enters the separating device; the mailpiece then may also stub.
Thus, one of the problems of the prior art is that high stack forces can cause stalling of mailpieces. Another problem of the prior art is that there can be poor separation of mail. Another problem of the prior art is that mail can be damaged by stubbing. Still another problem of the prior art is that multifeeds can occur when feeding mail. Yet another problem of the prior art is that stack forces can cause increased wear on feed rollers.